The present invention relates to a method for making a shaped ceramic composite article, and more particularly to a method for making a shaped composite comprising a ceramic matrix and refractory inorganic fiber reinforcement wherein the shaped article is of varying thickness.
As noted in U.S. Pat. No. 4,314,852, hot-pressing has been the most commonly used method for fabricating ceramic matrix composites comprising reinforcing fibers. That method typically comprises first providing a preform or "prepreg" material comprising a long or continuous fiber reinforcement impregnated with a finely divided matrix material in the form of a glass or ceramic powder, forming the impregnated fiber into sheets, and then cutting and stacking the sheets to provide a prepreg stack or preform which can be consolidated into a dense composite by hot pressing. The product of this process is typically a flat or curved planar article of substantially uniform cross-section.
To form more complex configurations from ceramic matrix composite materials, methods other than hot pressing have been proposed. U.S. Pat. No. 4,464,192, for example, describes an injection molding method wherein chopped fibers or whiskers are mixed with glass powders, the mixture is heated to the softening point of the glass, and is then injected into a mold having a desired shape. In U.S. Pat. No. 4,428,763, a transfer molding method for making fiber-reinforced glass composites is disclosed wherein long fibers are aligned in a mold cavity in predetermined orientation and a glass billet is then heated and transferred as molten glass into the mold.
While permitting the fabrication of more complex shapes, injection and transfer molding processes such as these have certain disadvantages. For example, where the fibers employed for reinforcement are chopped fibers or whiskers, they are difficult to align preferentially in directions requiring high strength in the molded product. And, where the loading of chopped fibers is high, the molten fiber/matrix material has high viscosity and is difficult to inject and form into complex shapes.
In the case of transfer molding, it is difficult to insure that the molten glass will completely encapsulate and surround the fibers in the mold cavity. Also, undesirable movement and/or breakage of the pre-aligned fibers in the course of glass transfer into the mold is difficult to avoid.
Attempts to modify hot-pressing procedures to adapt them to the fabrication of complex or precision composite parts have thus far likewise been unsuccessful. A significant problem in this respect resides in the nature of the prepreg materials which are available. As previously noted, these materials are powder-impregnated fibers or fiber tows of very low green density. Sheet stacks of this material constitute particularly unwieldy preforms for precision part forming, since preform densities are typically not more than 25% of the theoretical density of the finished part. This means that substantial debulking of the preform as well as consolidation to full density must be accomplished in a single forming event.
It is not unusual, even in fiber-reinforced polymer systems of much greater proportional starting density, for prepregs to be consolidated in a multi-step process. This is useful for precision part manufacture in order to maintain control over part configuration as the systems are debulked to their final density. For glass and glass-ceramic matrix composites, however, such an approach is not feasible due to factors such as processing expense, fiber degradation on repeated heating to the high forming temperatures required, and surface contamination of the composites from high temperature mold release aids.
It is therefore a principal object of the present invention to provide an improved method for the manufacture of fiber reinforced ceramic matrix composite articles wherein products of arbitrary shape and cross-sectional thickness profile may be made.
It is a further object of the present invention to provide a hot-pressing method for molding ceramic matrix composite parts wherein precision molded shapes comprising interior or exterior edge segments of precise thickness and adequate fiber density may be made.
It is a further object of the invention to provide a method for making ceramic matrix composite articles of improved edge finish.
It is a further object of the invention to provide refractory ceramic matrix composite articles of complex surface configuration and cross-section having improved internal and surface fiber distribution and physical integrity.
Other objects and advantages of the invention will become apparent from the following description thereof.